This invention relates to systems for batch weighing a fluent product and more particularly to a digital control for such systems.
In the batch weighing of fluent products including granular, pelletized, and other products which tend to flow, an advantageous type of weighing is known as bulk and dribble weighing. This involves delivery of the product to a scale bucket or the like at two different delivery rates, i.e., a first rate called the bulk delivery weight and a second and reduced rate called the dribble delivery rate. Initially, feed at both the bulk and dribble delivery rates is initiated to fill the scale bucket relatively rapidly. Then, when a first weight of product has been delivered to the scale bucket, the bulk delivery is terminated and product is delivered at the slower or dribble rate until the final weight of product has been attained. The scale bucket is then dumped to deliver the batch of product for filling a bag or other container and the next weighing operation is commenced.
Various control systems have been devised for overseeing bulk and dribble feed operations. These systems typically employ either mechanical or electrical controls for terminating feed operations at appropriate points to insure both speed of operation and weighing accuracy. Mechanical control systems are disadvantageous because they require mechanical adjustments which are time consuming and require trial and error to insure proper settings; because they include pick-up weights which are often inaccessible and therefore difficult to adjust and head indicators which are so positioned as to be difficult to read; because feed cut-off points are confined to a relatively narrow range of weights; because no record of production weights are made; and because cleanup is difficult.
Electrical control systems have in the past been analog systems in which some type of sensor (e.g., a strain gage or differential transformer) provides an input to which the system responds. Such analog systems are disadvantageous because sensor outputs are affected by variations in input voltage, temperature and humidity, as well as by aging. Also, because the sensor output is usually a low level voltage, it has to be amplified to a level compatible with the control circuitry. This gives rise to weighing inaccuracies as a result of amplifier drift and creates the expense of including drift compensation circuits in the system. Further, the input provided by a sensor must be converted from an analog to a digital signal which requires circuitry to perform the conversion and which leads to weighing inaccuracies arising from the attempt to convert a value representative of a percent of full scale to an equivalent digital signal. Finally, as in mechanical systems most sensor's outputs also only cover a narrow range of weights.